The present invention relates generally to body tissue fixation systems, including body tissue fixation hardware comprising biocompatible, bioabsorbable (resorbable), osteoconductive or osteoinductive plates, and methods of using those systems and hardware.
Traditional orthopedic and traumatological fixation systems to facilitate bone fracture healing (osteosynthesis) typically employ metallic hardware, e.g., plates, screws, rods and the like, formed of biocompatible, corrosion resistant metals such as titanium and stainless steel. Typical metallic plates are described e.g. in the book F. Sxc3xa9quin and R. Texhammar, AO/ASIF Instrumentation, Springer-Verlag, Berlin, Heidelberg, 1981, at p. 21-22, 55-79, 107-108, 117-122, the entire disclosure of which is incorporated herein by reference. While such systems are generally effective for their intended purposes, they possess a number of inherent shortcomings. For example, metal release to the surrounding tissues has been reported. See, e.g., L.-E. Moberg et al. Int. J. Oral. Maxillofac. Surg. 18 (1989) at pp. 311-314. Other reported shortcomigs include stress shielding, see P. Paavolainen et al., Clin Orthop. Rel. Res. 136 (1978) 287-293), and growth restriction in young individuals, see K. Lin et al., Plast. Reconstr. Surg. 87 (1991) 229-235. In infants and young children there is the risk that metallic plates and screws can sink into and below the cranial bone as a consequence of skull bone growth, thereby threatening the brain. See, e.g., J. Fearon et al., Plast. Reconstr. Surg. 4 (1995) 634-637. Therefore, it is generally recommended that non-functional implants should be eventually removed, at least in growing individuals. See C. Lindqvist, Brit. J. Oral Maxillofac. Surg. 33 (1995) p. 69-70.
Especially in maxillofacial and in cranial surgery, metallic mini plates are popular for use. See e.g. W. Muhlbauer et al., Clin. Plast. Surg. 14 (1987) 101-111; A. Sadove and B. Eppleg, Ann. Plast. Surg. 27 (1991) 36-43; R. Suuronen, Biodegradable Self-reinforced Polylactide Plates and Screws in the Fixation of Osteotomies in the Mandible, Doctoral Thesis, Helsinki University, Helsinki, 1992, p. 16 and references therein. Mini plates are small, thin, narrow plates, which have holes for screw fixation. They are typically located perpendicularly over a bone fracture to affix the bone mass on either side of the fracture to each other. Typical geometries of mini plates are described e.g. in U.S. Pat. No. 5,290,281 at FIGS. 6A-6F.
The main advantage of metallic plates (like titanium, stainless steel and cobalt chrome molybdenum plates), is that they are strong, tough and ductile so that they can be deformed or shaped at room temperature in an operating room, either by hand or with special intruments, to a form corresponding to the surface topography of the bone to be fixed. In this way, the plate can be fixed flush on the bone surface to which the plate is applied.
In light of the above shortcomings of metallic plates, however, bioabsorbable plates have been developed for fracture fixation. Longitudinal, six-hole plates were developed for orthopaedic animal studies. See Eitenmxc3xciller et al. (European Congress on Biomaterials, Abstracts, Instituto Rizzoli, Bologna, 1986, p. 94). However, because of their inadequate strength, some of the plates were broken in animal experiments involving fracture fixation.
A special advantage of bioabsorbable plates is that they can be provided with openings for the insertion therethrough of surgical fasteners (like screws), while allowing means to permit the formation of additional fastener openings therethrough during a surgical procedure at the surgeon""s discretion, as has been described in European Patent specification EP 0 449 867 B1.
Some bioabsorbable plates can be deformed (bended) permanently and safely only at elevated temperatures - above their glass transition temperature (Tg)xe2x80x94as has been described e.g. in EP 0 449 867 B1 and in U.S. Pat. No. 5,569,250. Below their Tg such plates are brittle and break easily when deformed. Only at temperatures above Tg does the molecular structure of such plates have enough mobility to allow shaping (e.g. bending), without the risk of breaking.
K. Bessho et al., J. Oral. Maxillofac. Surg. 55 (1997) 941-945 describes a bioabsorbable poly-L-lactide miniplate and screw system for osteosynthesis in oral and maxillofacial surgery. In order to be shaped, the plates of that reference must first be heated by immersion in a hot sterilized salt solution or by the application of hot air until they become plastic. Only after such heating can they be fitted to the surface of the bone.
EP 0 449 867 B1 describes a plate for the fixation of a bone fracture, osteotomy, arthrodesis, etc., said plate being intended to be fixed on bone with at least one fixation device, such as a screw, rod, clamp or corresponding device, wherein the plate comprises at least two essentially superimposed plates to provide a multilayer plate construction. The individual plates of said multilayer plate construction are elastic and flexible, so as to permit a change of form of said multilayer plate construction to substantially assume the shape of the bone surface in the operation conditions by means of an external force such as by hand and/or by bending instrument directed to said multilayer plate construction, whereby each individual plate assumes the position of its own with respect to other individual plates by differential motion along the respective surfaces of coinciding plates.
U.S. patent application Ser. No. 09/036,259 describes a bioabsorbable (bioresorbable or biodegradable), self-reinforced and/or oriented osteosynthesis plate which is strong, tough, and does not produce a substantial inflammatory response. The plate can be deformed, yet is dimensionally stable at temperatures below the glass transition temperature (Tg) of the material from which the device is made (e.g. at room temperature), thereby facilitating shaping. Such a bioabsorbable osteosynthesis plate is also dimensionally stable in tissue conditions, when fixed on bone surface to facilitate non-problematic bone fracture healing.
However, the above descibed metallic or bioabsorbale plates are not osteopromoting (osteoconductive or osteoinductive), which means that they do not actively promote new bone formation. Therefore, a need exists for a bioabsorbable plate which is osteoconductive and/or osteoinductive.
Partial osteoconductivity or osteoinductivity can be included into bioabsorbable plates by mixing osteoconductive or osteoinductive ceramic particles or fibers (made, e.g., of bioactive glass, of calcium phosphate or of hydroxyapatite) into a polymer matrix. Such materials are described e.g. in U.S. patent application Ser. No. 09/036,259. Osteoinductive properties can also be achieved, e.g., by mixing bone morphogenic proteins or osteoinductive demineralized bone into a polymer matrix.
However, in the materials described above, only a part of the plate surface can be osteoconductive or osteoinductive, because the osteopromoting particles and/or fibers can be mixed with the polymer matrix only in a limited amount, typically up to 40 wt-%. Because ceramic materials are typically 2-3 times heavier than polymers, this means that, in practice, the ceramic particle or fiber phase, in the materials mentioned above, cover only about 20% of the surface of the plate, at a maximum.
In this invention, the osteoconductive and/or osteoinductive character of bioabsorbable polymeric or composite plates is improved significantly by coating at least one surface of the plate with osteoconductive and/or osteoinductive (i.e., osteopromoting) particles or fibers or fiber fabric.
Accordingly, the present invention describes bioabsorbable materials and implants, like plates, that have at least one surface with an osteopromoting coating of ceramic or organic particles or fibers or fiber fabric, which coating additionally intensifies, guides and improves new bone formation. The osteosynthesis plate of the present invention includes an elongated section having a top face and a bottom face, which elongated section is shaped to traverse a fracture site or osteotomy site for subsequent fixation to an adjacent bone. The osteosynthesis plate further includes either on its top face or on its bottom face or on both faces an osteopromoting coating formed of ceramic or organic particles or fibers or fiber fabric. The osteosynthesis plate further may include a plurality of fastener openings disposed between the top face and bottom face to allow the traverse of a plurality of surgical fasteners therethrough. The osteosynthesis plate further may include areas disposed upon the elongated section to permit the formation of additional fastener openings therethrough during a surgical procedure at the discretion of the surgeon.
One advantage of the present invention is that it provides a biocompatible implant, like a plate, of sufficient strength to securely affix a plurality of adjacent bone portions and additionally to improve new bone formation along at least one osteoconductive and/or osteoinductive plate surface. Preferably, the biocompatible implant is bioabsorbable over a desired period of time without generating a substantial inflammatory response.
In a preferred embodiment, the bioabsorbable plates can be deformed either at room temperature or at an elevated temperature, and substantially retain their deformed (shaped) form in vivo so that they, e.g., keep bone fragments essentially in the desired position to facilitate bone fracture healing and/or new bone formation. In another preferred embodiment, the plates of the present invention are low-profile and oriented (either uniaxially or biaxially) to improve their strength characteristics, so that they are strong, yet deformable. Such oriented osteosynthesis plates of the present invention may be repeatedly deformed and returned to their original configuration at room temperature in order to contour the osteosynthesis plate precisely to a desired configuration through successive iterations.
The present invention also includes bioabsorbable fixation devices or surgical fasteners, like bone screws, that are capable of being inserted through fastener openings disposed within the osteosynthesis plates of the present invention. As such, the present invention contemplates a bone stabilization device including a bioabsorbable osteosynthesis plate and one or more bioabsorbable surgical fasteners.
The present invention also provides a method for forming a biocompatible, bioabsorbable, osteopromoting osteosynthesis plate, including the steps of forming a sheet, optionally orienting the sheet, either uniaxially or biaxially, coating at least one surface of the sheet with an osteopromoting coating, and forming an osteosynthesis plate from the sheet.
The present invention is also directed to a method for securing a plurality of adjacent bone portions, including the steps of providing a bioabsorbable, biocompatible, osteopromoting, osteosynthesis plate, positioning the osteosynthesis plate upon a plurality of adjacent bone portions, providing a plurality of surgical fasteners for securing the osteosynthesis plate to the adjacent bone portions, positioning the plurality of surgical fasteners through the osteosynthesis plate and securing the plurality of surgical fasteners into the adjacent bone portions.